In ligament replacement operations it is common practice to use staples for securing the ends of a tendon or equivalent tie to bone surfaces. These staples may take the form of simple U-shaped pieces of metal similar to paper staples except that they are generally of greater thickness and strength. In some instances, the staples may comprise a top with four depending legs at the four corners of the top thereby providing a four-prong staple.
One problem encountered with the use of orthopedic staples of the above type, particularly the four-prong staple, is the driving in of two of the legs in the same cleavage plane or "grain direction" of the bone. In this respect, the situation is similar to the driving of two nails in alignment with a grain of wood which has a tendency to split the wood. The same result can occur in a bone if the staple legs are in alignment with a cleavage plane; that is, fall in the same plane. In this respect, the cleavage planes of a bone are similar to the "grain" of wood. Splitting of the bone results in a weakened gripping of the staple legs and a possibility of the tendon or other tie becoming disconnected from the bone as a result of pulling out of the staple.
In other situations, even where a staple is firmly imbedded in the bone, there can still occur a pulling out or disconnection of the ligament or equavelant tie from the staple. In this respect, tension on the ligament can ultimately pull the stapled end out from beneath the top of the staple.
Orthopedic staples of the two-prong type are shown in my U.S. Pat. No. 4,467,478, issued Aug. 28, 1984 which patent is concerned with a human ligament replacement.